The invention relates to the sidewalls and the beads of tires and, more particularly, the cushion compounds in contact with the reinforcing cords of the carcass or carcasses anchored in the beads.
The beads of tires are rigid parts which function to seat the tire on the rim and anchor the carcasses. Their contact with the rim, particularly the rim flanges, must be made, during travel, with a minimum of slippage to reduce wear due to rubbing against the rim. The sidewalls, on the other hand, are very flexible. It is important to avoid any sudden variation in rigidity between the beads and the sidewalls, particularly to ensure good endurance of the tires. Thus the beads may be considered to comprise two parts, an anchoring zone whose walls are designed to come into contact with the seat and the flange of the rim and, radially outwardly, a transition zone adjacent to the sidewalls. The anchoring means of the carcasses are arranged in the anchoring zone of the bead.
European Patent 0,582,196 discloses a tire comprising a tread extended by two sidewalls and two beads, as well as a carcass anchored in the two beads and having an annular reinforcement. The carcass is made up of cords arranged in adjacent fashion, aligned circumferentially and in contact with at least one layer of cushion compound having a very high modulus of elasticity in the anchoring zone of the bead that contains the annular reinforcement. In this tire, the annular reinforcement of the anchoring zone of the bead is made up of piles of circumferential cords with a layer of cushion compound of high modules or elasticity interposed between the reinforcing cords of the carcass and said piles.
The mode of anchoring of the carcass of this tire has the advantage of avoiding a sudden variation in rigidity due to the turn-up end of the carcass. The change in rigidity between the anchoring zone of the carcass and the sidewalls can thus be much more gradual.
The subject of the invention is a tire in which the structure of the beads and the sidewalls is further perfected to improve its quality and performance without complicating its manufacture.
In the following, xe2x80x9ccordxe2x80x9d is understood to mean both monofilaments and multifilaments, or assemblies such as cables, twists or any other type of equivalent assemblies, and this, regardless of the material or treatment of these cords, for example, surface treatment or coating or preglueing to promote adhesion to the rubber.
xe2x80x9cLayer of cushion compoundxe2x80x9d for a given reinforcing ply is understood to mean the rubber blend in contact with the reinforcing cords of the ply, adhering to these and filling the interstices between adjacent cords. In current industrial practice, for a given carcass ply, the same quality of cushion compounds is used for the various zones of the tire.
xe2x80x9cContactxe2x80x9d between a cord and a layer of cushion compound is understood to mean that at least part of the outer circumference of the cord is in intimate contact with the rubber blend constituting the cushion compound.
xe2x80x9cSidewallsxe2x80x9d are the regions of the tire having little flexural strength situated between the crown and the beads. xe2x80x9cSidewall blendsxe2x80x9d are the rubber blends situated axially outwardly in relation to the reinforcing cords of the carcass and to their cushion compounds. These blends have a low modulus of elasticity. The sidewalls usually have no reinforcement other than the carcass.
The part of the tire adjacent radially inwardly to the sidewall is called the xe2x80x9cbead.xe2x80x9d Two zones or regions may be distinguished in a bead, an xe2x80x9canchoring zonexe2x80x9d whose radially inner and axially outer walls are designed to come into contact, when the tire is in use, with a seat and a flange of the rim and, radially outwardly, a xe2x80x9ctransition zonexe2x80x9d adjacent to the sidewall. The anchoring means of the carcasses are also arranged in the anchoring zone. These means are usually an annular reinforcement which may be a rod, piles of circumferential cords or any other means.
The transition zone of the bead constitutes an intermediate part between the anchoring zone and the sidewall whose flexural rigidity diminishes progressively as the sidewall is approached. This region comprises, in particular, bead filling rubbers called xe2x80x9cbead fillersxe2x80x9d. Conventionally in this transition zone of the bead a sidewall blend constitutes the outermost protection against mechanical and atmospheric attacks. It would be advisable for the demarcation between the transition zone of the bead and the sidewall to correspond to the radially outer limit of the fillers. The demarcation between the transition and anchoring zones of the bead corresponds radially to the radial height of the flange of the rim on which the tire is designed to be mounted. However, when annular reinforcing elements go beyond this radial limit, it would be advisable that the limit between the transition and anchoring zones of the bead correspond to the radially outer limit of the annular reinforcement anchoring the carcass.
xe2x80x9cModulus of elasticityxe2x80x9d of a rubber blend is understood to mean a modulus of secant extension obtained at a deformation of uniaxial extension of about 10% at ambient temperature.
The tire according to the invention comprises a tread extended by two sidewalls and two beads, each bead having an anchoring zone and a transition zone, as well as a carcass anchored in the two beads, the said carcass being made up of cords. This tire is characterized in that, in at least part of the said transition zone of the beads, the reinforcing cords of the carcass arranged axially furthest toward the outside in the tire are in contact, on the axially outward side of the said cords, with a layer of cushion compounds having a given modulus of elasticity and the reinforcing cords of the carcass arranged axially furthest inwardly in the tire are in contact, on the side toward the inside of the tire, with a layer of cushion compounds having a lower modulus of elasticity and in that, in at least part of the said sidewalls, the reinforcing cords of the carcass arranged axially furthest outwardly in the tire are in contact, on the side of the said cords toward the outside of the tire, with a layer of cushion compounds having a modulus of elasticity lower than that of the said layer of cushion compounds having a given modulus of elasticity and the reinforcing cords of the carcass arranged axially furthest inwardly in the tire are in contact, on the side toward the inside of the tire, with a layer of cushion compounds having a modulus of elasticity lower than that of the said layer of cushion compounds having a given modulus of elasticity.
Preferably, the layer of cushion compounds having a given modulus of elasticity has a modulus of elasticity which may range from medium to very high.
The layer of cushion compounds having a given modulus of elasticity situated in the transition zone of the beads on the axially outer side of the tire in relation to the carcass reinforcing cords has the advantage of cooperating with said reinforcing cords to obtain a high circumferential rigidity of the whole. Such high rigidity helps to limit slippage between the outer wall of the bead and the rim flange during travel without penalizing the endurance of the bead on the inner side of the reinforcing cords of the carcass where the layers of cushion compounds are subjected to appreciable stresses of radial extension. This results in a marked gain in endurance of the tire.
According to a preferred embodiment of the invention, in at least part of the transition zone of the bead the layers of cushion compounds in contact with the reinforcing cords of the carcass arranged axially furthest outwardly in the tire, on the side of the said cords toward the outside of the tire are made up of at least one bead filler blend axially adjacent the outer side of the said reinforcing cords.
This embodiment provides a complementary role for the bead fillers concerned, that of rubber bonding with the reinforcing cords of the carcass. Reduction of the diversity of blends simplifies manufacture of the tire without sacrificing performance due to the proposed architecture of the tire.
Advantageously, the layers of cushion compounds in contact with the reinforcing cords of the carcass on the side of the said cords toward the outside of the tire are made up of at least two bead fillers having unlike moduli of elasticity and adjacent axially outwardly to the said reinforcing cords, the zone of contact between the bead fillers having a lower modulus of elasticity and the reinforcing cords being staggered axially outwardly in relation to the zone of contact between the bead fillers having a high modulus of elasticity and the reinforcing cords.
The bead fillers having a lower modulus of elasticity and adjacent outwardly to the reinforcing cords of the carcass may have a medium or high modulus of elasticity. The bead filler having a higher modulus of elasticity and adjacent outwardly of the reinforcing cords of the carcass may have a high or very high modulus of elasticity.